I have a rebuilt 2.1 in my 87 GL Wolfy that is running rich.  I don't seem to have any problems other than poor mileage.  It has about 8000 miles on it and gets only 15 miles to the gallon mix city/highway.  I starts up great cold or warm, has good power for a waterboxer, and doesn't stumble.  The tailpipe is dark black and sooty water drips out of the tailpipe during idle.  It also burns oil at about 1 quart per thousand miles.  I don't know what to do about the rich running.  The whole engine was a top notch rebuild job and has new plugs, wires, O2 sensor, no vacuum leaks, and has the timing set accurately.

I am seem to get a swing with a .5 volt center on my o2 senser at idle so I don't know what the problem is.
 
Thanks for any assistance you all can offer.  I used to get 19-20 miles per gallon with 150,000 miles on the engine.